The subject of the present invention is a method for reducing the glare of a receiver of a system, especially a geolocation system, the system furthermore comprising a plurality of emitters.
One of the emitters is for example a pseudolite. The expression “pseudolite” designates a terrestrial emitter transmitting signals which have the same structure as the signals dispatched by a satellite. The term “pseudolite” corresponds to the contraction of the term “pseudo-satellite”. For example, a GPS (global positioning system) pseudolite emits a signal at 1.57542 GHz phase-modulated by a Gold code and a navigation message. The expression “Gold code” designates a pseudo-random binary sequence for example discussed in the article “Optimal binary sequences for spread spectrum multiplexing” by Robert GOLD.
A Gold code is the result of combining two time-shifted maximal length sequences. Maximal length sequences are themselves periodic binary sequences generated by shift registers (of 10 bits for GPS and 9 bits for the Russian GLONASS system). The properties of maximal length sequences are as follows: they are balanced, that is to say the number of 1s in the code is equal to the number of 0s plus 1 and, if N is the size of the sequence, the autocorrelation equals −1/N away from the main peak.
The invention applies especially to systems using code-based multiplexing, also called “code division multiple access” (CDMA), this being for example the case for GPS and for GNSS (global navigation satellite system).
The phenomenon of glare (“near far”), also called the phenomenon of intrinsic interference, is a major problem occurring in systems using CDMA, the signals being emitted on the same frequency. When the codes used by the emission sources do not exercise sufficient discrimination with respect to the difference in power which may exist between the sources on reception by a receiver, this glare or intrinsic interference phenomenon occurs. When the receiver is dazzled by glare, it is no longer capable of tracking the weakest code, even by making errors. The Gold codes, used in GPS, allow a discrimination of generally between 23.9 dB and 60.2 dB between two signals originating from two distinct sources. Thus, provided that two signals have more than some twenty or so decibels of deviation in power, interference phenomena may occur.
A recently envisaged application for GPS relates to the guidance of aircraft during the landing and/or takeoff phases, this involving the deployment of a system comprising one or more emitters, for example pseudolites, to improve the precision of the vertical coordinate of the aircraft, as well as at least one receiver. As long as the airplane remains sufficiently far from the runway in proximity to which the pseudolite is placed, the extra signal emitted by the pseudolite behaves as a conventional satellite signal, but, as soon as the airplane approaches the runway, the signal of the pseudolite becomes so powerful that it dazzles the receiver through glare, so that the latter may not detect the signals arising from the other emitters, which are for example satellites.
In such a system, the receiver receives for example the signals coming from the pseudolite and the satellites simultaneously and correlates the signal resulting from this reception with a local replica, also subsequently called the “local signal”, of the signal emitted by one of the emitters of the system and that it wishes to track. If the signal that it wishes to track has the lowest power, the peaks in the cross-correlation between the local signal and the other more powerful signals may take values which may disturb the main correlation peak of the tracked signal, or indeed even jam it totally in the case where the tracked signal is particularly attenuated with respect to the other signals.
A known solution for solving this problem of glare consists in using pseudolites whose emission is pulsed, as explained for example in the publication “GPS pseudolites: theory, design and applications” H. Stewart Cobb or in the work “Global positioning system: theory and applications” Bryant D. ELROD A. J. VAN-DIERENDONCK. Nonetheless, such a technique may not turn out to be sufficient and be relatively complex to implement.
Other techniques have also been proposed for solving the problem of glare mentioned hereinabove, such as frequency shifting, also called “frequency offset”, or else frequency jumping, also called “frequency hopping”.
There exists a need to have a method for reducing the glare of at least one receiver within a system comprising several emitters, which is relatively simple to implement, effective and inexpensive.